Known material handling machines, such as excavators, have a material handling arm assembly. The arm assembly may have a first arm, known as a boom, pivotally mounted about a generally horizontal axis relative to a chassis of the machine. A second arm, known as a dipper, may be attached to an end of the boom remote from the chassis and may be pivotable about a generally horizontal axis. A material handling implement, such as a bucket, may be pivotally mounted on an end of the dipper. The boom may be raised and lowered by operation of a first hydraulic ram. The dipper may be movable relative to the boom by operation of a second hydraulic ram. The bucket may be movable relative to the dipper by operation of a third hydraulic ram.
It is known to retrofit actuated attachments, for example clamps, known as thumbs, to material handling machines. Such clamps or thumbs may be pivotally mounted adjacent to the bucket on an end of the dipper. The clamp or thumb may be used to grip against the bucket to pick up objects, for example rocks or tree trunks. The clamp or thumb may be movable to lie against the dipper when not in use. The clamp or thumb may be movable relative to the bucket and the dipper by operation of a fourth hydraulic ram. The fourth hydraulic ram may be mounted on the dipper remote from the bucket and the clamp or thumb. The fourth hydraulic ram may be mounted on the dipper via a mount or bracket that is welded to the dipper. In particular the mounted bracket may be welded to a face of the dipper arm, which face may be the furthest point from a neutral axis of the dipper arm. The weld used to weld the mounted brackets onto the dipper arm may therefore be the first weld on that face of the dipper arm.
The fatigue life of dipper arms to which mounted brackets are welded is reduced by a combination of the stress concentration effect of welding and the size and weight of the mount or bracket on the dipper. The highest and lowest points of dippers in such modified material handling machines experience increased stresses, even when the clamp or thumb is not in use, i.e. when the bucket is being used to pick up and move material.
The impact of these effects can be reduced by providing reinforced or heavier dippers, however this increases the amount of material required to construct the material handling machine and results in heavier material handling machines.